1. Field of the Invention
The present invention relates to a reduced video signal processing circuit for displaying a reduced input video scene in a window, using a buffer memory, such as a field memory, a frame memory, etc., wherein the reduction ratio is changeable.
2. Description of the Prior Art
A Picture in Picture (PIP) function has been generally known as a TV display function for displaying a plurality of scenes in the form of a window on one TV screen, instead of displaying one scene on the entire TV screen. With the arrival of the multimedia era, a further variety of display functions have been demanded. In particular, a window displaying function at a desired reduction ratio, which has become commonly used as an operating environment for personal computers, has also been demanded for TV displaying. In order to display a separate scene in the form of a window, a buffer memory, such as a field memory, a frame memory, etc., is necessary in order to retain synchronism between main and sub-scenes (inset-scenes).
FIG. 6 is a block diagram showing a conventional reduced video signal processing circuit.
A video signal for a sub-scene which is displayed in the form of a window is input into an input processing section 1 which includes a filter circuit 10. The input video signal is then processed for size reduction (filtered) by the filter circuit 10 according to size reduction ratio data K before being provided to field memories 2 and 3. A writing operation to the field memories 2 and 3 is controlled by an input video clock generator 5 in a control block 4. The control block 4 is also provided with a display video clock generator 6 for controlling a reading operation from the memories 2 and 3. Size reduction ratio data K, one externally received, and the input video clock generator 5 supplies the data K to the input processing section 1.
Based on a horizontal synchronizing signal (Input H) and vertical synchronizing signal (Input V) of an input video signal received, the input video clock generator 5 outputs a write clock (WCLK), write enabling signals (WE1, WE2), and a write reset signal (WRST). A WCLK has the same rate as that of a pixel clock synchronous with an input H. WE1 and WE2 signals cause the field memories 2 an d 3 to be at an enable state alternately for every field during an effective display period of an input video signal. The field memories 2 and 3 incorporate address counters for independently addressing during writing and reading operations. After the counters are reset by a WRST signal, addresses of the counters are incremented by counting a WCLK while WE1 and WE2 signals are at an H (high) level, whereby a reduced video signal supplied from the input processing section 1 is written into the field memories 2 and 3. When size reduction ratio data K indicates "1," that is, when a reduction operation is not executed, WE1 and WE2 signals remain at an H level throughout an effective video period. When size reduction ratio data K is smaller than "1," on the other hand, the period when WE1 and WE2 signals are at an H level is adjusted according to the size reduction ratio data K. For instance, with size reduction ratio data K of "1/2, " as shown in FIG. 7, WE1 and WE2 signals are output at an H or L level in such a way that they become an H level for every other pixel. As a result, an input video signal subjected to half thinning processing in the filter 10 can be written into a memory.
Further, the input video clock generator 5 computes video size data (SIZ), based on the size reduction ratio data K, and supplies the SIZ data to the display video clock generator 6. For instance, provided that the numbers of horizontal and vertical pixels of an input video signal are "640" and "480," respectively, and the size reduction ratio data K is "1/2," SIZ data is computed to indicate H (horizontal) SIZ "320" and V (vertical) SIZ "240."
On the read side, in order to read a reduced video signal from the field memories 2 and 3, the display video clock generator 6 is supplied with a horizontal synchronizing signal (display H) and a vertical synchronizing signal (display V) of a display video signal for a main scene and a display position data (X, Y) for indicating a display position of a reduced scene, and outputs a read clock (RCLK), read enabling signals (RE1, RE2), and a read reset signal (RRST). RCLK has the same rate as that of a pixel clock synchronous with a display H. RE1 and RE2 signals cause the field memories 2 and 3 to be at an enable state alternately for every field during an effective display period of a display video signal. An RRST signal resets read address counters of the field memories 2 and 3 upon a rise of an RE signal. After the counters are reset by an RRST signal, a read address of the counters in the field memories 2 and 3 are incremented by counting an RCLK while RE1 and RE2 signals are at an H level, whereby a reduced video signal is read from the field memories 2 and 3. Note that the display video clock generator 6 may generate display H and V signals in cases where each timing thereof is previously known, so that an RRST signal, an RCLK, RE1 and RE2 signals, etc., are generated based on the display H and V signals.
FIG. 3 illustrates an input video signal A displayed in the form of a window, wherein display position data (X, Y) indicates a display position of a sub-scene with respect to a main scene (a display video signal), and video size SIZ data (H, V) indicates the size of a sub-scene to be displayed in the form of a window (a reduced video signal generated from an input video signal). For achieving such a window display, the display video clock generator 6 causes RE1 and RE2 signals to be at an H level only during an effective display period as shown in FIG. 7, based on the SIZ data (H, V) and the video position data (X, Y). In this case, RE1 and RE2 signals are continuously maintained at an H level throughout an effective display period, which is different from the input side.
The display video clock generator 6 supplies SIZ data (H, V) and video position data (X, Y) to a display processing section 7 which is provided downstream of the field memories 2 and 3, so that the reduced video signal read from the memories 2 and 3 are processed therein for window displaying through framing or addition of background data, and output as a display video signal.
In order to change a reduction ratio as desired in the foregoing procedure, the content of processes executed on the write and read sides must be changed in accordance with respective new reduction ratio data. However, if the content of a process is changed during reading and writing operations, distortion may be caused to a display video signal (displaying scene). Thus, an operation for changing a reduction ratio is executed during a vertical blank interval of a video to prevent distortion.
However, since an input V and a display V are not synchronous with each other, the above changing operation is conducted at different timings on the write and read sides. An example is taken, referring to FIG. 7, where a phase of a display V is delayed compared to that of an input V. When size reduction ratio data K is newly input at time T1, the reduction ratio is changed to the new ratio during a vertical blank period NP1 which is an immediately following interval of the time T1 in the input video clock generator 5 and the input processing section 1, so that a reduction operation and a write control operation are thereafter conducted based on the new reduction ratio. In the display video clock generator 6 and the display processing section 7, on the other hand, the reduction ratio is changed during a vertical blank period DP1 which is also an immediately subsequent interval to the time T1. Since this timing (DP1) is behind the changing timing on the input side (NP1), a video signal written in the changed reduction ratio is read from the memory in the changed reduction ratio after the time DP1.
In another case, referring to FIG. 8, where reduction ratio data is newly input at time T2 which is after the vertical blank interval NP1 of an input V and before the vertical blank interval DP1 of a display V, a reduction ratio is changed during a vertical blank interval NP2 after the time T2 in the input video clock generator 5 and the input processing section 1, and during a vertical blank interval DP1, before the interval NP2, in a display video clock generator 6 and the display processing section 7. In other words, the reduction ratio is changed on the display side prior to the input side. As a result, a video signal reduced in a previous reduction ratio (a reduction ratio before the change) is read in a changed reduction ratio to be displayed for a field subsequent to the change (DP1) on the read side. This causes significant distortion to a video displayed in a window. For this reason, the prior art has a problem in that a reduction ratio cannot be changed while a window display continues.